Heat exchangers used to recover heat from such power plant exhaust gas are often somewhat large and cumbersome in design. Consequently they are often designed to be transported in component form and assembled on site. Additionally they are not optimized efficiently for space and straightforward connection to the gas turbine or gas/diesel engine. These design limitations lead to the requirement for additional floor-space and increased transportation, assembly, testing and maintenance costs. These difficulties sometimes lead to operators opting for simple cycle (no heat recovery), which is considerably less efficient than a combined cycle and in which hot exhaust gas is vented straight to atmosphere. Historically simple cycle power plants may have been installed when there were fewer environmental concerns and fuel consumption was not critical.
Further problems are also recognized in the industry. Irregular flow distribution in power plant exhaust gas delivered to the heat exchanger (for example a velocity of 120 m/s forward flow to a backflow of 20 m/s in the same duct) can cause damage to heat exchange tubes, linings, dampers, burners and other plant equipment. Damage may be caused by excessive vibration, oscillations, or the like. The standard remedy has been to provide longer ducts with increased cross sectional area to allow the higher velocities to reduce naturally over distance. Again however this results in inefficient use of space and increased costs. Alternatively the components may be made significantly stronger and more durable, but this requires more expensive materials and manufacturing and increases weight.
It is sometimes desirable for heat exchangers to convert extra heat energy, when compared to the amount of heat present in the exhaust leaving the power plant, in order to increase the output of the heat exchange process. In presently used systems this is often achieved by a duct burner, which heats the hot exhaust gases further after they have left the power plant and before they enter the heat exchanger. The amount of extra output that can be gained in this way is however limited; the exhaust gases are already at a relatively high temperature which may be close to the maximum temperature tolerance of the heat exchanger, linings and internal components.
According to a first aspect of the invention there is provided a heat exchange unit arranged to be used to recover energy from exhaust gas. The heat exchange unit generally comprises a gas inlet duct to which a heat exchange duct is connected. A heat exchange array of a heat exchange system may be situated within the heat exchange duct and may surround a maintenance duct. The maintenance duct may be arranged to allow access for inspection and/or maintenance of at least part of the heat exchange system.
The maintenance duct may conveniently replace a by-pass duct where this is not required (for example where the heat exchange unit is a steam generator). The maintenance duct may allow for maintenance and/or inspection to be carried out on the heat exchange system in a controlled environment, without the need for the heat exchange unit to be located within a building. The maintenance duct may be sized in order to allow man access thereto; for example it may be sized to allow a man to enter the maintenance duct and inspect the inside thereof.
According to a second aspect of the invention there is provided a heat exchange unit arranged to be used to recover energy from exhaust gas. The heat exchange unit may comprise an inlet duct to which a heat exchange duct is connected. A heat exchange array may be situated within the heat exchange duct and the inlet duct and heat exchange duct may have substantially perpendicular longitudinal axes so as in use gas is delivered to the heat exchange duct in a direction substantially perpendicular to the longitudinal axis of the heat exchange duct.
If the inlet duct and heat exchange duct have substantially perpendicular longitudinal axes so as in use gas is delivered to the heat exchange duct in a direction substantially perpendicular to the longitudinal axis of the heat exchange duct, advantages over alternative systems may be evident. In some current systems at least part of an inlet duct is provided with a significant curve to allow connection to an end of a heat exchange duct which is substantially perpendicular to the remaining part of the inlet duct. The present system may be more straightforward than this prior art system and may allow for easier and closer connection between the source of the exhaust gas and the heat exchange duct.
According to a third aspect of the invention there is provided a heat exchange unit arranged to be used to recover energy from exhaust gas. The heat exchange unit may comprise an inlet duct to which a heat exchange duct is connected. At least two heat exchange arrays may be situated within the heat exchange duct and between the at least two heat exchange arrays is a heating mechanism.
The heating mechanism may be a burner or electrical elements for example.
Such an arrangement may allow for enhanced heat conversion. This may be particularly useful where an increase in the heat conversion may be required despite a potential loss in efficiency arising from the consumption of additional fuel in the heating mechanism.
It will be appreciated that any one of the first, second and third aspects may be combined with one or both of the other aspects. With this in mind the following embodiments may be combined with one or more of the aspects described above, where the features discussed in said embodiments are also present in said aspect or combination of aspects.
Where a maintenance duct is provided it may be substantially cylindrical. In view of the heat exchange array, this may provide a space-efficient solution whereby the maintenance duct provides enough room for access, but does not necessitate an unnecessary increase in the size of the heat exchange unit.
In some embodiments the heat exchange duct and maintenance duct are substantially coaxial. Again this may provide a space efficient solution whereby the heat exchange duct and heat exchange array necessitate only the minimum required increase in the size of the heat exchange unit.
In some embodiments pipes and headers for supply to and/or exit from the heat exchange array are provided in the maintenance duct.
In some embodiments the maintenance duct provides access to the pipes and headers for their inspection and maintenance. In this way inspection and maintenance can be carried out in a controlled environment (e.g. without inclement weather hampering the work). Additionally the maintenance duct may mean that access to the pipes and headers is significantly improved.
In some embodiments the maintenance duct is provided with a vertical access means for passing substantially the full height of the maintenance duct. Thus a ladder or lift for example may be provided inside the maintenance duct to assist with inspection and/or maintenance.
In some embodiments the maintenance duct provides structural support for the heat exchange unit. This may reduce or eliminate the structural load placed on the heat exchange duct, which may facilitate flexibility with regard to materials used and the design of the heat exchange unit as a whole.
In some embodiments the maintenance duct acts as a deflector for gas entering via the gas inlet duct, so as to alter the gas flow distribution. This may help to improve gas flow distribution.
In some embodiments the gas inlet duct is provided with at least one duct burner. This may allow for enhanced heat conversion in the heat exchange unit.
In some embodiments the gas inlet duct is positioned so as to introduce the gas tangentially to a portion of the interior perimeter of the heat exchange duct. This may improve flow distribution and reduce back pressure. Specifically tangential gas entry may create high speed circulating gas currents which dissipate their kinetic energy in a controlled manner, before moving through the heat exchange duct.
In other embodiments the gas inlet duct is positioned so as to introduce the gas so that the gas impinges upon a splitter within the gas inlet duct. A portion of the maintenance duct may provide the splitter.
In some embodiments first and second heat exchange arrays and the heating mechanism are positioned so as exhaust gas falls to a temperature between 250° C. and 350° C. before reaching the heating mechanism. In some embodiments the two heat exchange arrays and the heating mechanism are positioned so as exhaust gas falls to a temperature of approximately 300° C. before reaching the heating mechanism.
Such arrangements may provide an efficient system. A large quantity of the thermal energy carried by the gas entering via the exhaust gas inlet duct is recovered by the first heat exchange array. Following this, at the temperatures discussed, the gas may still be sufficiently hot (with the given oxygen content in the gas) to allow combustion in the heating mechanism. The heating mechanism may then re-heat the gas to proximate the maximum safe temperature tolerance of the heat exchange unit, linings and internals, whereupon the second heat array recovers the thermal energy from the re-heated gas. The first heat exchange array may also help remove turbulent flow from the exhaust gas in order that the flow is more regular when it reaches the or each heating mechanism. The skilled person will appreciate that turbulent flow can cause problems with such heating mechanisms and potentially extinguish flames therefrom.
In some embodiments the heating mechanism raises the temperature of the exhaust gas to between 700° C. and 800° C. In some embodiments the heating mechanism raises the temperature of the exhaust gas to approximately 760° C. These temperatures may be proximal to the maximum temperature tolerance of materials such as stainless steel which may be used in the heat exchange unit.
In some embodiments the heating mechanism is a ring burner. In view of its shape a ring burner may be particularly appropriate where the heat exchange duct is cylindrical (has a circular cross-section).
In some embodiments the gas inlet duct is not provided with a burner. This may allow for the gas inlet duct to be shorter, thus potentially decreasing the distance between the source of the exhaust gas and the heat exchange unit, making the whole system more space efficient.
In some embodiments the heat exchange array(s) is helical. Such a shape is convenient since it allows for a compact run of tubes. However, other forms of array may equally be possible.
In Some Embodiments the Exhaust Gas is Produced by a Gas Turbine.
In some embodiments the heat exchange unit is a once through steam generator. As will be appreciated embodiments of the present invention may provide a space efficient solution to heat recovery. Use with a once through steam generator (also a space efficient technology) may therefore be advantageous in order that the overall system has a small footprint.
In some embodiments the heat exchange unit is substantially weather proof. This may be advantageous as it may not then be necessary to house the heat exchange unit within a building. Additionally this may make inspection and maintenance of the heat exchange unit easier and safer.
In some embodiments the heat exchange unit is roughly between 2.6 m and 8 m in diameter.
In some embodiments the heat exchange duct is substantially cylindrical. This may be especially suitable in view of the use, in some embodiments, of one or more helical heat exchange arrays, and may provide a space efficient solution.
In some embodiments, when installed, the heat exchange duct is arranged substantially vertically. This may make the heat exchange duct (and heat exchange unit in general) more suitable for replacing any existing exhaust stack. Additionally it may reduce the footprint of the heat exchange duct.
According to a forth aspect of the invention there is provided a method of re-fitting a process heat source unit (exemplified by a simple cycle gas turbine), so as to convert it to combined cycle, the method comprising the steps of:
1) providing a heat exchange unit arranged to recover energy from exhaust gas, the heat exchange unit comprising an inlet duct to which a heat exchange duct is connected, wherein a heat exchange array is situated within the heat exchange duct;
2) delivering the heat exchange unit, which is generally pre-assembled and tested, to the location of the process heat source unit; and
3) replacing an existing exhaust stack of the process heat source unit with the heat exchange unit.
In some embodiments the process heat source unit is a gas turbine.
In some embodiments foundations used for supporting the existing exhaust stack are used to support the heat exchange unit. This may reduce costs and the time necessary for conversion.
In some embodiments the inlet duct and heat exchange duct have substantially perpendicular longitudinal axes so as in use gas is delivered to the heat exchange duct in a direction substantially perpendicular to the longitudinal axis of the heat exchange duct This may reduce the height of the heat exchange duct. It may also reduce the time necessary for conversion as a perpendicular inlet duct may be less complicated and more easily structurally supported than for example a co-axial inlet duct.
The method may utilise a heat exchange unit according to any of the above aspects of the invention.